Cermet resistance element and material



June 20, 1967 Wl E, COUNTS ETAL 3,326,645

CERMET RESISTANCE ELEMENT AND MATERIAL Filed Sept. 22, 1965 FIG. 4

INVENTORS WILLUAM E. COUNTS WILLHAM T. KELLY FIG. e 3%1/ Y United StatesPatent O 3,326,645 CERMET RESIISTANCE ELEMENT AND MATERIAL WilliamEdward Counts, Anaheim, and William Thomas Kelly, Garden Grove, Calif.,assignors to Beckman lnstruments, liuc., a corporation of CaliforniaFiled Sept. 22, 1965, Ser. No. 489,337 4 Claims. (Ci. 29--l82.5)

The present invention relates to an improved electrical resistancematerial formed of a mixture of finely divided particles of glass andmetal, of the type commonly called cermet material, and to resistanceelements constructed therefrom.

Cermet resistance elements presently known in the art are exemplified byU.S. Patent 2,950,995 of Thomas M. Place, Sr., et al., entitledElectrical Resistance Element and 2,950,996 of Thomas M. Place, Sr., etal., entitled Electrical Resistance Material and Method of Making Same,both of which are assigned to Beckman Instruments, Inc., assignee of thepresent invention. These patents describe a resistance element formed ofa layer of resistance material comprising a heterogeneous mixture ofparticles of non-conducting material and conducting metals xed to anon-conducting base. The non-conducting material is a ceramic-typematerial such as glass or vitreous enamel and the layer is formed byheating the metal-glass mixture at least to the melting point of theglass or enamel, so as to create a smooth, glassy phase.

In brief, the present invention resides in the discovery that a cermetmixture, formed of finely divided particles of glass and an alloy ofabout to 50% total weight of ruthenium and rhodium, in which the metalalloy is so correlated to the glass material that the minimumproportions of ruthenium metal is at least by weight of the metal alloyfor all proportions of the alloy weight, can be deposited on anon-conductive base member and red to form a cermet resistance elementhaving a high resistivity and a suitable temperature coefficient ofresistance. Such resistance elements have a high resistance for therelatively large amounts of metal material employed and are capable ofperformance at high power ratings as compared to glass-metal resistanceelements employed in the past.

A more thorough understanding of the invention may be obtained by astudy of the following detailed description taken in connection with theaccompanying drawings in which:

FIG. 1 is an isometric view of an embodiment of the invention which issuitable for use in rotary potentiometers;

FIG. 2 is an isometric view of another embodiment of the invention whichis suitable for use in linear potentiometers as well as for fixedresistors;

FIG. 3 is an isometric view of an embodiment of the inventionparticularly adapted for fixed resistance elements of amicro-miniaturized electrical circuit;

FIGS. 4 and 5 are sectional views taken along lines l-i and 5 5respectively of FIG. 3, and

FIG. 6 is an underside view of the embodiment of PIG. 3.

The cermet material of the present invention is particularly applicableto the manufacture of miniature or micro-miniature resistance elementsof all types, representative ones being shown in the gures. Thus, in thestructure of FIG. l, a layer l0 of resistance material is fired to abase lll, the electrodes l2, 13 being provided at each end of the layerfor connecting into an electrical circuit. This resistance element maybe used as a fixed resistor or may be combined with a rotating contactarm for use as a rotary rheostat or potentiometer. The base lll may 'beof any suitable electrically non-conducting roce material which Willwithstand the elevated temperatures normally used to fire the resistancematerial. Various ceramic materials are suitable for this use, thosehaving a smooth, tine-textured surface and being impervious to moistureand other liquids being preferred. Steatite, fosterite, sintered orfused aluminas and zircon porcelains are examples of preferred materialsfor forming the base 1l.

The electrically conductive electrodes i2, 13 are conventional and maybe formed by applying any of the Wellknown conducting silver or othermetal pastes over the layer of resistance material and firing the unitto convert the paste to a layer of metal which is firmly attached to thelayer of resistance material. Alternatively, terminal structures such asare shown in Patent No. 3,134,085 of Kenneth F. Miller et al., entitledVariable Resistor With Terminal Structure may be employed for makingelectrical contact with the resistance layer l0.

FIG. 2 illustrates another form of the resistance element of theinvention in which a layer l5 of resistance material is applied to -arectangular base 16 and electrodes 17, 18 are then added at the ends ofthe layer l5. This form of the invention is particularly suitable infixed resistors and linear potentiometers.

FIGS. 3, 4, 5 and 6 illustrate substantially enlarged views of amicro-miniature circuit element 20 advantageously formed of alumina andsupporting a pair of fixed cermet resistance elements 21. The respectiveends of these resistance elements engage conductive electrodes 22 asshown.

The invention contemplates a cermet composition for use in theabove-described resistor elements comprising about 50% to about 95%glass and the balance an al-loy of the metals ruthenium and rhodium. Theparticular amounts of ruthenium :and rhodium necessary in the metalalloy to produce a resistance layer having a commercially applicabletemperature coefficient of resistivity depends on the total metalcontent employed in the cermet composition. For relatively highresistance elements, in the order of 10,000 ohms/square, the range ofproportions of ruthenium content may be about 95% by Weight of the totalmetal content of the cermet compositions. In general, the maximumpreferred allowable proportion of ruthenium decreases as the total metalcontent of the metal-glass cermet is increased. Preferably, therutheniumrhodium containing alloy is so related to the ceramic materialthat when the total content of metal alloy in the cermet composition is540% the maximum proportion of ruthenium metal is about 95% by weight ofthe alloy and the `maximum proportion of rhodium is about 40% lby weightof the alloy; when the total content of the metal alloy in the cermetcomposition is between lil-20% of the composition, the maximumproportion of ruthenium metal is about 90% by Weight of the alloy andthe maximum proportion of rhodium is :about 55% by Weight of the alloy;when the total metal content of the cermet composition is between 20-35of the composition, the maximum proportion of ruthenium metal is about70% by weight of the alloy and the maximum proportion of rhodium isabout 60% by weight of the alloy; when the total metal content isbetween S55-50%, the maximum proportion of ruthenium metal is about 65%by weight of the alloy and the maximum proportion of rhodium is about byweight of the alloy.

In a preferred embodiment of the invention, the cermet compositions ofthis invention may be prepared by mixing the resinates of rutheniumtogether with the resinates of rhodium. The glass binder, in the form offinely divided glass particles, is mixed or milled with the resinatesolution so that each glass particle is thoroughly wetted with the metalsolution. This mixture is gradually heated to approximately 700 F. andconstantly stirred to remove the volatiles and organic materials fromthe mixture and t decompose the metal compounds. The resulting drymaterial is ground to a line powder and calcined at about 850 F. Theresulting calcine is ground to a fine powder, producing a dry materialconsisting of very small glass particles coated with or mixed withextremely small particles of the metal alloy.

The particular range of proportions of glass to metal by weight in theiinal resistance material may Ibe varied in the procedure just describedby varying the amount of glass added to a given resinate solution. Eachof the individual resinate solutions contains a predetermined quantityby weight of ruthenium-rhodium alloy. After heating the glass and metalresinate solution, only the glass and metal remain, the total weight oramount of metal particles coating the glass particles Ibeing that amountof metal which was originally in the resinate solutions.

The mixtures formed by the method described above may be storedindefinitely and may be used to produce stable resistance elements. Whenit is desired to make resistance elements using the material, the drypowder is mixed with a suitable liquid carrier, eg., 7% ethylhydroxyethyl cellulose-93% octyl alcohol to form a fluid compositionwhich can be applied -to the basev member by any suitable process suchas silk screening, spraying or stenciling. The base with the layer orlayers applied thereto is then tired to drive off the volatiles and fusethe glass material into a continuous phase of solidiiied glass with themetal alloy particles uniformly distributed therein. One tiringprocedure comprises inserting the base with applied resistance layer orlayers into a cold furnace. The furnace is slowly heated, eg., over aperiod of four hours to a temperature lof the order of 1450' to 1550 F.This temperature is maintained a short time period, eg., 20 to 30minutes, after which the furnace is slowly cooled, eg., over a period offour hours before withdrawal of the cermet elements. This procedure isreferred to below as the slow firing technique. Another tiring procedurewhich may be used (denoted hereinafter as the fast ring method) involvesinserting the base of they tired resistance layer or layers into afurnace which has been pre-heated to a temperature in the range of1400-2000" F. The `fired element is then removed -after a relativelyshort time period, eg., to 30 minutes.

For the purpose of giving those skilled in the art a betterunderstanding of the invention, the following illustrative examples aregiven for various cermet resistance elements using ruthenium-rhodiumalloys tired onto a steatite base member to form a thin resistance layerapproximately .001 inch in thickness. In these examples, it will beunderstood that the time and temperature of ring and the type ofsubstrate material employed may cause some minor variations `in theresistivity and temperature coeiiicient of the resulting resistanceelements.

Example A (5% metals by weight) z Percent `Glass 95 Ruthenium 3.33Rhodium 1.67

The resistivity of a resistance l-ayer of approximately .001 inchthickness formed of the material was approximately 10,000 ohms/ squareand its temperature coeflicient of resistivity (TEMPCO) was 338 p.p.m./C.

Example B (10% metals by weight): Percent Glass 90 Ruthenium 6.67

Rhodium 3.33

The resistivity of a resistance layer of approximately .001 inchthickness formed of this material was approximately 1900 ohms/square andits TEMPCO was 160 p.p.m./ C.

4 Example C (10% metals by weight):

Glass, percent 90 Ruthenium, percent 5 Rhodium, percent 5 Resist-ance,ohms/ squ-are 8000 TEMPCO, p.p.m./ C. -738 This composition produced aresistance layer having a TEMPCO outside the more desired commerciallyapplicable range of i500 p.p.m./ C. In order to produce a resistanceelement and within the more commercially applicable TEMPCO range, it isdesirable to limit the maximum proportions of rhodium in thoseresistance materials having a metal content of between 5-10% to about40% of the total metal content.

Example D (l5 metals thy weight) TEMPCO, p.p.m./ C 420 Example F (20%metals by weight):

Glass, percent Ruthenium, percent 12 Rhodium, percent 8 Resistance,ohms/ square 166 TEMPCO, p.p.II1./ C -885 Example G (20% metals byweight):

Glass, percent 80 Ruthenium, percent l0 Rhodium, percent l0 Resistance,ohms/square 272 `TEMPCO, p.p.m./ C -458 Example H (20% metals byweight):

Glass, percent 80 Ruthenium, percent 6.67 Rhodium, percent 13.33Resistance, ohms/square 1055 TEMPCO, p.p.m./C. 681

This composition produced a resistance element having a TEMPCO outsidethe more desired commercially applicable range of i500 p.p.m./ C. Inorder to produce a resistance element within the more commerciallyaplicable TEMPCO range, it is desirable to limit the maximum proportionsof rhodium in those resistance materials having a metal content ofbetween 20-35% by Weight to about 60% of the total metal content.

Example I (20% metals by weight):

Glass, percent 80 Ruthenium, percent 17.14 Rhodium, percent 2.86Resistance, ohms/square TEMPCO, p.p.m./ C '+245 Example J (25% metals byweight):

Glass, percent 75 Ruthenium, percent 12.50 Rhodium, percent 12.50Resistance, ohms/square TEMCO, p.p.m./ C. -98

Example K (25% metals by weight):

Glass, percent 75 Ruthenium, percent 11.18 Rhodium, percent 13.82Resistance, Ohms/ square TEMPCO, p.p.m./ C. 428

3,326,645 e? Y Example L (30% metals by weight): TEMPCO within thecommercially applicable desired Glass, percent 70 range of i500 p.p.m./C. This is over 34 times more Ruthenium, percent resistance usingapproximately the same quantity of metal Rhodium, percent l5 in thecermet composition. The resistance of the cermet Resistance, ohms/square68 5 composition utilizing 50% ruthenium-rhodium alloy is TEMPCO,p.p.m./ C. -208 still approximately 30 ohms per square which isextremely Example M (35% metals by Weight): high when one considers thatthe total metal content is Glass, percent 65 approximately five timesthat of the above-described gold, Ruthenum, percent palladium, silvercermet composition. Rhodium percent 15 10 While .it is not completelyunderstood why the rutheni- Resistance ohms/square 3 5 tim-rhodium metalalloy can be employed in such large TEMPCO, ppm/o C +101 quantities toform a resistance material, it is believed that the presence ofruthenium inhibits the tendency for the Example N (35% metals byWelght): metal to agglomerate and form strings or globules in the GlassPercent e5 15 fused glass binder. It appears that ruthenium contributesRuthemum percent e- 15'55 to the interaction between the respectivesurfaces of the Rhedlum Percent 19'45 finely divided metal alloy andglass particles and prevents Reslstance ohms/segnare 83 theagglomeration thereof even when they are present in TEMPCO PP'm'/ C'*295 relatively large quantities. This apparently causes the mix-Example 0 (40% metals by weight): 20 ture to retain its homogeneityduring firing and results in a Glass, percent 60 much more stableresistance element having relatively Rutlienium, percent `20 largequantities of metal for the resistance values ob- Rhodium, percent 20tained. Resistance, ohms/square 32 The homogeneity of resistanceelements formed of TEMPCO, p.p.m./ C. 42.8 25 cermet composition usingruthenium-rhodium alloys is Example P (50% metals by Weight): believedto be one reason why these elements are capable Glass, percent 50 ofwithstanding higher power levels. Because the materials Rutheniumpercent do not agglomerate when the glass material is fused and,

Rhodium percent 25 therefore, do not produce strings or globules ofmetal llo the current liow throu h the resistance element is Resistance,ohms/square 32 a .y g TEMPCO, ppm/n C. l +124 believed to he moreuniformly applied .across the entire a volume of the resistance element.That is, the metal par- Vaflatmns 111 the glass COmPOSlUOIl UtlllZed mayChange ticles are uniformly distributed throughout the element meresistance and TEMPCO Values achieved le 'a mmof and there are no areasor regions through which electrical extent. It is believed that almostany glass composition 35 current 50W is confined to a Single or narrowpath may be employed S0 long ae me glass eenslmems d0 not through anyportion of the element. This, in addition to reeel Wltll the metaleenstltuems at the lmg tempera' the greater metal content of theresistance element, creates tures employed and S0 long 'as the glass hasa melting tem' a more even distribution and dissipation of the heatperature below that of the metal constituents of the mixproduced. ture"Tlle palllcular eemposltlens of the glass Were em" e0 The relativelyhigh metal content of cermet elements Pleyed m Venous quanmles m theabove examples' formed of ruthenium-rhodium cermet material and theextremely uniform distribution of the metal alloy through- Glassformulae Glass #l Glass #2 Glass #3 out the element is extremelyimportant in another respect. One of the major problems encountered whenusing cermet Pmgfs Pemfgl 10 Fermi?) 20 45 resistors as variableresistance devices, such as precision 5:41 5:41 potentiometers andcircuit trimmers, is the high electrical Mgg ------lnoisecharacteristics associated with such devices due to i65i 245o 2000 thecontact made between the movable wiper of such 240 1-20 devices and theresistance element. It is believed that this 50 electrical noise is atleast in part due to the somewhat sporadic contact made by the wiper andthe metal par- The above formulae are the fritted percentages. Theticlcs on the surface of the element. Due to the high metal glass may beproduced by any conventional process. lIt is content -of cermet elementsemploying ruthenium-rhopreferred, however, that it be as homogeneous aspossidium alloys for relatively high resistance elements, and Ible. Onemethod of making glass includes thoroughly miX- because the metalparticles do not agglomerate but are ing a batch of the raw materialstogether while dry, meltevenly distributed throughout, it can beunderstood that ing the batch in ceramic crucibles to produce a clearfluid there are substantially greater numbers of metal particles glass,quenching the molten glass by pouring it into cold available for contactwith the wiper. This greatly reduces Waef, drying the resultingShattered glass and then gfldthe electrical noise associated with suchelements. ing it to a very line powderg Resistance elements formed ofcermet compositions As will be noted from the examples, relatively largequantities of ruthenium-rhodium alloys can be utilized in these cermetcompositions to form `resistance elements having relatively highresistance values as compared to cermet resistance elements produced inthe past. It is known, for example, that an alloy of gold, palladium andsilver, when utilized in a glass-metal (cermet) resistance mixture offinely divided particles in which the gold, pal ladium, silver contentis about 11.5% of the total mixture, produces a resistance elementhaving approximately 55 ohms per square for a layer approximately .001inch in thickness. As will be seen in Example B hereinabove, a cermetcomposition using 10% ruthenium-rhodium alloy produces a resistanceelement (of like thickness) having a resistance of 1900 ohms per squareand having a using ruthenium-rhodium cermet material also have anextremely smooth surface. This is considered extremely important whensuch elements are employed in variable resistance devices, such aspotentiometers. These ruthenium-rhodium cermet materials do not produceblisters or other irregularities on the surface of the element when theminimum percentage of ruthenium in the 4composition is at least 10% Iofthe total metal alloy content in the composition. For example, in orderto produce smooth surfaced resistance elements, the ruthenium contentshould not be less than .5% of the total composition when the totalmetal content is 5% of the composition and should not be less than 5% ofthe composition when the total metal alloy content is 50% of thecomposition. Of course, it will be understood that the above minimumpercentage of ruthenium does not necessarily produce a resistanceelement having a temperature coecient of resistivity falling within thedesired commercially applicable range of i500 p.p.m./ C. and, for themore desired range, greater percentages of ruthenium must be employed.

Although exemplary embodiments ofthe invention have been disclosed anddiscussed, it will be understood that other applications of theinvention are possible and that the embodiments disclosed may besubjected to various changes, modifications and substitutions withoutnecessarily departing from the spirit of the invention.

What is claimed is:

1. A cermet composition adapted to be red Aonto a non-conducting basemember to form a thin resistance layer consisting essentially of:

about 50 to 95% by Weight of glass material; and

to 50% by weight of an alloy of ruthenium and rhodium in which the metalalloy is so correlated to the glass material that the minimum proportionof ruthenium metal is at least 10% by weight of the metal alloy for allproportions of the alloy content, said glass material having a meltingpoint lower than that of said alloy of ruthenium and rhodium.

2. A cermet composition adapted to be tired onto a nonconducting basemember to form a thin resistance layer consisting essentially of:

about to 95 by weight glass material; and

about 5 to 50% by weight of an alloy of ruthenium and rhodium, saidglass material having a melting point lower than that of said alloy ofruthenium and rhodium, said alloy constituents being so co-related tosaid glass material that when the total metal content of saidcomposition is between 5-l0%, the maximum proportion of ruthenium isabou-t 95% by weight of the alloy and the maximum proportion of rhodiumis about 40% by Weight of the alloy; when the total metal content isbetween 10-20% of the composition, the maximum proportion of rutheniumis about 90% by weight of the alloy and the maximum proportion ofrhodium is about by weight of the alloy; when the total metal content isbetween 1Z0-35% of the composition, the maximum proportion of rutheniumis about 70% by weight of the alloy and the maximum proportion ofrhodium is about by weight of the alloy; and when the total metalcontent is between 3550% of the composition, the maximum proportion ofruthenium is about by weight of the lalloy yand the maximum proportionof rhodium is about 75% by weight of the alloy.

3. A cermet composition adapted to be tired onto a nonconducting basemember to form a thin resistance layer consisting essentially of:

Iabout 50 to 95 by weight glass material; and

about 5 to 50% by weight of an alloy of the metals ruthenium and rhodiumin which the maximum proportions of ruthenium metal t-o total metalcontent is inversely proportional to the total metal content and themaximum proportion of ruthenium is about 95% by Weight of the alloy whenthe metal content is 5% and the maximum proportion of ruthenium in thealloy is about 65% when the total content is 50% by weight of the cermetcomposition, said glass material having a melting point lower than thatof said alloy of ruthenium and rhodium.

A cermet resistance element comprising high temperature resistant,electrically non-conducting base;

a thin layer of fused glass-metal mixture tired to said base, said layerconsisting essentially of 50 to 95% by weight of solidified glass and 5to 50% by weight of an alloy of the metals ruthenium and rhodium in nelydivided form homogeneously mixed throughout said fused glass, said glasshaving -a melting point lower than that of said alloy of ruthenium andrhodium, said metal alloy being so co-related to said glass constituentsthat when the total metal content of said composition is between 5-10%,the maximum proportion of ruthenium is about 95 by weight of the alloyan-d the maximum proportion of rhodium is about 40% of the alloy; whenthe total metal -content is between itl-20% of the composition, themaximum proportion of ruthenium is about 90% by weight of the alloy andthe maximum proportion of rhodium is about 55% by weight of the alloy;when the total metal content is between 20-35% of the composition, themaximum proportion of ruthenium is about by weight of the alloy and themaximum proportion of rhodium is about 60% by weight of the alloy; andwhen the total metal content is between 35-50% of the composition, themaximum proportion of ruthenium is about 65% by weight of the alloy andthe maximum proportion of rhodium is about by weight of the alloythereby providing a resistance element having a resistivity of 30 to10,- 000 ohms per square when said layer is approximately .001 inch inthickness and a temperature coecient of resistivity of less than i500p.p.m./ C.

are

)References Cited UNITED STATES PATENTS FOREIGN PATENTS 8/ 1965 GreatBritain.

CARL D. QUARFORTH, Primary Examiner.

55 BENJAMIN R. PADGETT, Examiner.

A. l. STEINER, Assistant Examiner.

` ent requiring correction and that t UNITED STATES PATENT oEFICECERTIFICATE OF CORRECTION Patent No 3 ,326 ,645 June 20, 1967 WilliamEdward Counts et al.

It is hereby certified that error appears in the above numbered pathesaid Letters Patent should read as corrected below.

Column 8, line 5, after "total" insert alloy Signed and sealed this 2ndday of July 1968.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

1. A CERMET COMPOSITION ADAPTED TO BE FIRED ONTO A NON-CONDUCTING BASEMEMBER TO FORM A THIN RESISTANCE LAYER CONSISTING ESSENTIALLY OF: ABOUT50 TO 95% BY WEIGHT OF GLASS MATERIAL; AND 5 TO 50% BY WEIGHT OF ANALLOY OF RUTHENIUM AND RHODIUM IN WHICH THE METAL ALLOY IS SO CORRELATEDTO THE GLASS MATERIAL THAT THE MINIMUM PROPORTION OF RUTHENIUM METAL ISAT LEAST 10% BY WEIGHT OF THE METAL ALLOY FOR ALL PROPORTIONS OF THEALLOY CONTENT, SAID GLASS MATERIAL HAVING A MELTING POINT LOWER THANTHAT OF SAID ALLOY OF RUTHENIUM AND RHODIUM.